78 



PLECTOMYCETES 



[CH. 



Erysiphales 



The Erysiphales are characterized by an abundant superficial mycelium, 

 which may be white (colourless) or dark-coloured. The perithecia are 

 spherical, ovoid or flattened, and are usually without an ostiole; the peridium 

 is thin and membranous; the asci are arranged in a regular layer at the 

 base of the perithecium. 



The group includes some 600 species, the majority of which are external 



parasites or epiphytes upon the 

 leaves of higher plants. They are 

 grouped into three families, of 

 which the Microthyriaceae are 

 but little known, and of doubtful 

 position, and the Erysiphaceae 

 and Perisporiaceae show several 

 points in common both with the 

 Plectascales, from which they 

 differ in the regular arrangement 

 of their asci, and with the Pyre- 

 nomvcctes, from which they are 

 for the most part distinguished 

 by the absence of an ostiole. 



Their taxonomic position is 

 probably somewhere between 

 these two groups, and they have, 

 under various systems of classifi- 

 cation, been placed in closer proxi- 

 mity sometimes to the one and 

 sometimes to the other. Their in- 

 clusion here in the Plectomycetes 

 is due to the fact that they, or 

 rather their best-known family, 

 the Erysiphaceae, show indica- 

 tions of being a primitive group. 

 The simple type of male and 

 female organs, the latter without 

 a trichogyne, and the simple 

 structure of the perithecium are evidence in this direction. 



The families of the Erysiphales may be distinguished as follows : 



Aerial mycelium colourless (or white). Perithecia more 



or less globose without an ostiole, furnished with 



conspicuous appendages. 



Conidia of oidium type. 



Fig. 37. Terfezia olbiensis Tul. ; section through 

 hymenium, showing asci irregularly arranged; 

 after Tulasne. • 



Erysiphaceae. 



